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A yawn (from the Middle Englishyanen, an alteration of yonen or yenen, which in turn comes from the Old Englishgeonian[1]), is a reflex of simultaneous inhalation of air and stretching of the eardrums, followed by exhalation of breath. Pandiculation is the act of stretching and yawning simultaneously.[2]

Yawning is associated with tiredness, stress, overwork, lack of stimulation, and boredom. Yawning can also be a powerful non-verbal message with several possible meanings, depending on the circumstances. In humans, yawning has an infectious quality (i.e., seeing a person yawning, talking to someone on the phone who is yawning, or just thinking of yawning can trigger yawning) which is a typical example of positive feedback.[3] Infectious yawning has also been noted in chimpanzees.

There are a number of theories that attempt to explain why animals yawn.[4][5] It is likely that there are a number of triggers—not just one—for the behavior. However, there are a few select theories that attempt to explain the primary evolutionary reason for the yawn. None of them have been empirically substantiated. The first states that yawning occurs when one's blood contains increased amounts of carbon dioxide and, therefore, becomes in need of the influx of oxygen that a yawn can provide.[4] Researchers believed this theory to be true at least since Hippocrates, but studies have since shown it to be either incorrect or, at the very best, flawed.[6] Yawning may, in fact, reduce oxygen intake compared to normal respiration, not increase it.[7]

The second notion states that yawning is the body's way of controlling brain temperature.[8] The process is thought to cool off the brain, much like a fan cools the inside of a computer.[9] Another speculated reason for yawning is the desire to stretch one's muscles.[10] Nervousness has also been suggested as a possible reason. There have been studies that suggest that yawning, especially for psychological reasons ("contagious" yawning), may have developed as a way of keeping a group of animals alert.[11] Anecdotal evidence suggests that yawning helps increase the state of alertness of a person – paratroopers have been noted to yawn in the moments before they exit the aircraft,[12] while speed skaterApolo Ohno yawns before races because it relaxes him.[13][14]

Contents

Proposed causes

Research data strongly suggest that neither contagious nor story-induced yawning are reliable in children below the age of six years.[15]

In 2007, researchers (a professor of psychology) from the University of Albany proposed that yawning may be a means to keep the brain cool. Mammalian brains operate best within a narrow temperature range. In two experiments, they demonstrated that both subjects with cold packs attached to their foreheads and subjects asked to breathe strictly nasally exhibited reduced contagious yawning when watching videos of people yawning.[16][17] A similar recent hypothesis is that yawning is used for regulation of body temperature.

Another hypothesis is that yawns are caused by the same chemicals (neurotransmitters) in the brain that affect emotions, mood, appetite, and other phenomena. These chemicals include serotonin, dopamine, glutamic acid, and nitric oxide. As more (or less) of these compounds are activated in the brain, the frequency of yawning increases. Conversely, a greater presence in the brain of opioidneurotransmitters such as endorphins reduces the frequency of yawning. Patients taking the selective serotonin reuptake inhibitors Paxil (paroxetine HCl) or Celexa (citalopram) have been observed yawning more often. Excessive yawning is more common during the first three months of taking the SSRI's. Anecdotal reports by users of psilocybin mushrooms often describe a marked stimulation of yawning while intoxicated, often associated with excess lacrimation and nasal mucosal stimulation, especially while "peaking" (i.e., undergoing the most intense portion of the psilocybin experience). While opioids have been demonstrated to reduce this yawning and lacrimation provoked by psilocybin, it is not clear that the same pathways that induce yawning as a symptom of opioid abstinence in habituated users are the mode of action in yawning in mushroom users. While even opioid-dependent users of psilocybin on stable opioid therapy often report yawning and excess lacrimation while undergoing this entheogenic mushroom experience, there are no reports in the literature of habituated users experiencing other typical opioid withdrawal symptoms such as cramping, physical pain, anxiety, gooseflesh, etc. on mushrooms

Yet another hypothesis is that yawns are caused by a lack of O2 in the bloodstream, and that yawn reflex incorporates a "rush of oxygen" to the brain.

Recent research carried out by Garrett Norris, a lecturer in psychology at the University of Leeds, involving monitoring the behavior of students kept waiting in a reception area, indicates a connection (supported by neuro-imaging research) between empathic ability and yawning. "We believe that contagious yawning indicates empathy. It indicates an appreciation of other people's behavioral and physiological state," said Garrett.[18]

Yawning behavior may be altered as a result of medical issues such as diabetes[19], stroke[20] and adrenal conditions.[21]

Contagiousness

The yawn reflex has long been observed to be contagious: in 1508, Erasmus wrote: "One man's yawning makes another yawn",[22] and the French proverbialized the idea to "Un bon bâilleur en fait bâillier deux." ("One good gaper makes two others gape").[23] Often, if one person yawns, this may cause another person to "sympathetically" yawn.[7][24] Observing another person's yawning face (especially his/her eyes), even reading, or thinking about yawning, or looking at a yawning picture can cause a person to yawn. [25][26][27] The proximate cause for contagious yawning may lie with mirror neurons, i.e., neurons in the frontal cortex of certain vertebrates, which upon being exposed to a stimulus from conspecific (same species) and occasionally interspecific organisms, activates the same regions in the brain.[28] Mirror neurons have been proposed as a driving force for imitation which lies at the root of much human learning, e.g., language acquisition. Yawning may be an offshoot of the same imitative impulse. A 2007 study found that young children with autism spectrum disorder do not increase their yawning frequency after seeing videos of other people yawning, in contrast to typically developing children. This supports the claim that contagious yawning is based on the capacity for empathy.[29]

To look at the issue in terms of evolutionary advantage, if there is one at all, yawning might be a herd instinct.[30] Other theories suggest that the yawn serves to synchronize mood in gregarious animals, similar to the howling of the wolf pack. It signals tiredness to other members of the group in order to synchronize sleeping patterns and periods. This phenomenon has been observed among various primates. The threat gesture is a way of maintaining order in the primates' social structure. Specific studies were conducted on chimpanzees[31] and stumptail macaques.[32] A group of these animals was shown a video of other conspecifics yawning; both species yawned as well. This helps to partly confirm a yawn's "contagiousness."

Gordon Gallup, who hypothesizes that yawning may be a means of keeping the brain cool, also hypothesizes that "contagious" yawning may be a survival instinct inherited from our evolutionary past. "During human evolutionary history when we were subject to predation and attacks by other groups, if everybody yawns in response to seeing someone yawn, the whole group becomes much more vigilant, and much better at being able to detect danger."[16]

A recent study by the University of London has suggested that the "contagiousness" of yawns by a human will pass to dogs. The study observed that 21 of 29 dogs yawned when a stranger yawned in front of them, but did not yawn when the stranger only opened his mouth.[33]

Non-human yawning

In animals, yawning can serve as a warning signal. For example, Charles Darwin, in his book The Expression of the Emotions in Man and Animals, mentioned that baboons yawn to threaten their enemies, possibly by displaying large, canine teeth.[34] Similarly, Siamese fighting fish yawn only when they see a conspecific (same species) or their own mirror-image, and their yawn often accompanies aggressive attack.[35]Guinea pigs also yawn in a display of dominance or anger, displaying their impressive incisor teeth. This is often accompanied by teeth chattering, purring and scent marking. Adelie Penguins employ yawning as part of their courtship ritual. Penguin couples face off and the males engage in what is described as an "ecstatic display," their beaks open wide and their faces pointed skyward. This trait has also been seen among Emperor Penguins. Researchers have been attempting to discover why these two different species share this trait, despite not sharing a habitat. Snakes yawn, both to realign their jaws after a meal, and for respiratory reasons, as their trachea can be seen to expand when they do this. Dogs often yawn after seeing people yawn, and when they are confused.[36]

Culture

Certain cultures ascribe superstitions to the act of yawning. The most common of these is the belief that it is necessary to cover one's mouth when one is yawning in order to prevent one's soul from escaping the body. The Ancient Greeks believed that yawning was not a sign of boredom, but that a person's soul was trying to escape from its body, so as to rest with the gods in the skies. This belief was also shared by the Mayans.[37]

A soldier hides his yawn from his lady companion in this detail from a painting by Oscar Bluhm titled Ermüdende Konversation, or "Tedious conversation".

Several superstitions have been concocted regarding the act of yawning and the harm that the act can do to the individual yawning. These superstitions may not only have arisen to prevent people from committing the faux pas of yawning loudly in another's presence – one of Mason Cooley'saphorisms is "A yawn is more disconcerting than a contradiction", and in 1663 Francis Hawkins advised "In yawning howl not, and thou shouldst abstain as much as thou can to yawn, especially when thou speakest"[38] – but may also have arisen from concerns over public health. Polydore Vergil (c. 1470–1555), in his De Rerum Inventoribus, writes that it was customary to make the Sign of the Cross over one's mouth, since "alike deadly plague was sometime in yawning, wherefore men used to fence themselves with the sign of the cross...which custom we retain at this day."[39] Muhammad, the last Prophet of Islam, is reported by Bukhari to have mentioned, "Yawning is from Satan and if anyone of you yawns, he should check his yawning as much as possible, for if anyone of you (during the act of yawning) should say: 'Ha', Satan will laugh at him."[40]

^Hawkins, FrancisYouth's Behavior, or, Decency in Conversation amongst Men (1663) quoted in Mencken, H. L.. A New Dictionary of Quotations on Historical Principles from Ancient and Modern Sources New York: Vintage, 1942